The present application relates to a roller truck for supporting a rotating object relative to a stationary object and/or for facilitating rotation of the rotating object relative to the stationary object. It finds particular application in the context of computed tomography (CT) scanners, such as might be used in medical, security, and/or industrial applications, for supporting a rotating gantry (e.g., comprising an x-ray source and a detector array) that rotates relative to a stationary portion of a CT scanner. For example, a roller truck may be positioned between a support frame (e.g., a stationary member) and a rotating gantry and may be configured to support the mass of the rotating gantry and/or to facilitate rotation of the rotating gantry relative to the support frame. It also relates to other applications where supporting a rotating object may be useful.
A typical CT scanner comprises a rotating gantry comprising a central opening (e.g., a bore) large enough to receive an object (e.g., a patient, luggage, etc.) extending along a scanning axis, and the rotating gantry is rotated about the object during an examination procedure. An x-ray source is positioned on the rotating gantry substantially diametrically across the central opening from a detector array. As the rotating gantry rotates, the x-ray source emits x-rays that traverse the object and are detected by the detector array. By rotating the x-ray source about the scanning axis and relative to the object, x-rays are projected through the object from a plurality of different directions. An image of an examined portion of the object can then be constructed from data yielded from the detected x-rays.
Because undesirable vibrations of the CT scanner during an examination can cause faulty or erroneous image information, the structure of the scanner (e.g., the rotating gantry and the support frame) typically comprises massively reinforced components often weighing a ton or more in an effort to reduce mechanical noise. Consequently, because of the weight, the rotating gantry has usually been supported in the frame by an expensive and heavy precision roller bearing or ball bearing assembly (e.g., sometimes referred to by those skilled in that art as a tank turret bearing).
Many of the disadvantages inherent in such massive, expensive, relatively fixed CT scan structures were recognized and addressed, at least in part, by the apparatus disclosed in U.S. Pat. No. 4,928,283 issued May 22, 1990 to Gordon, which is assigned to the assignee of the present disclosure. In the aforesaid '283 patent, Gordon broadly suggests the use of rollers rather than bearings for rotatably supporting the rotating gantry in a support frame, without however discussion of the nature and characteristics of such rollers.
U.S. Pat. No. 5,473,657 issued Dec. 5, 1995 to McKenna, which is also assigned to the assignee of the present disclosure, discloses an improved x-ray tomography structure comprising a support frame configured to support the rotating gantry. The mass of the rotating gantry rests on one or more resilient rollers so that the top half of the rotating gantry is substantially unconstrained (e.g., for centerless rotation) so as to allow the rotating gantry to expand and contract while introducing little to no error-producing stresses into the rotating gantry and/or support frame. The one or more rollers are mounted in a truck that is generally supported about a pivot axis. The resilient rollers serve to dampen the transfer of vibrations to the rotating gantry as it rotates and/or accommodate temperature cycling of the rotating gantry.
While the '657 patent provided significant cost and weight benefits over the use of bearings, the rotation of the rotating gantry and/or minor imperfections in the rollers and/or rotating gantry continued to generate and/or transfer vibrations, or vibration frequencies, which could potentially interfere with the operational frequencies of the CT scanner and which could result in degradation to images yielded from a CT examination. Additionally, it was determined that the ability of the rollers to dampen the transfer of vibrations was limited because the amount of energy required to move the rotating gantry directly depended upon the dampening (e.g., the greater the dampening, the more energy required).
In U.S. Pat. No. 6,823,037 issued Nov. 23, 2004 to Riemer et al., which is also assigned to the assignee of the present disclosure, it was purposed that the vibration frequencies be shifted to frequency ranges in which the CT scanner normally did not operate. To do this, two or more roller trucks were proposed, with at least one of the trucks comprising a spring plate and at least another of the trucks not comprising the spring plate. In this way, the vibration frequencies of the CT scanner would be less likely to interfere with the operational frequencies of the CT scanner, for example.
However, merely shifting vibration frequencies may be inadequate for current performance expectations of CT scanners. For example, 20 years ago, the CT scanners generally operated at merely one or two relatively slow speeds, or frequencies. Today, CT scanners are expected to rotate at much higher RPMs and, in some applications, a single CT scanner may be expected to rotate at a plurality of different speeds (e.g., within a wide range of RPMs). Thus, it has become difficult to shift the vibration frequencies to a frequency range that will not interfere, or will not substantially interfere, with the operational frequencies of the CT scanner.